Sea lice are regularly monitored and counted on fish at a salmon farm. © cermaq.com
The salmon louse Lepeophtheirus salmonis is an ectoparasite linked to declines in wild salmonid populations as well as causing huge economic losses in salmon farms. Previous studies, using a variety of molecular markers, yielded conflicting results ranging from strong genetic differentiation among nearby farms to no structure across the entire North Atlantic.
Besnier et al. (2014) investigated the effects of anthropogenically-driven rapid evolution to pesticide resistance using a SNP-array. Emamectin benzoate, or EMB, is the most commonly-used pesticide to control L. salmonis in the Atlantic.
[EMB] resistance developed at a single source, and rapidly spread across the Atlantic [within a decade] … and importantly demonstrates that alleles conveying resistance to pesticides may be quickly spread over very large areas in the marine environment.
From a management perspective, this study demonstrates the necessity of ocean-wide policies, rather than management at the regional level.
From a population genetic perspective, the seascape in which L. salmonis is evolving is extremely heterogeneous:
with patches of high host density in salmon farms and coastal areas, and large areas of low host density in the offshore regions.
Thus, standard popgen protocols with which to investigate different evolutionary dynamics may be difficult to apply due to biased estimates in these sea louse population(s).
Finally, from an evolutionary perspective, the strong selective sweeps detected in this study strongly suggest that:
L. salmonis has a high capacity to spread new advantageous mutations across [ocean basins] in the time scale of just a few generations [max 11 years] … thus corroborating concerns that pesticide resistance can develop and rapidly spread over large areas on an ecological time-scale.